Hi,
I have observed the following behavior in the following scenario.

var files

vars/static.yml
config_names:
- value1
- value2

vars/test.yml
config_names:
- value3
- value4

playbook

vars_prompt:
- name: "filename"
prompt: "Filename"
private: no
when: filename is not defined
vars_files:
- vars/static.yml
- [ vars/ {{ filename }}. yml, vars/static.yml ]

pre_tasks:
- debug: msg="{{ item }}"
with_items: config_names
roles:
- { role: test, with_items: config_names }
post_tasks:
- debug: msg="{{ item }}
with_items: config_names

Execution 1:

  ansible-playbook -i test.yml playbook.yml -e 'filename=test"

Result:
No prompt.
vars/test.yml would loaded in order
pre_tasks, roles, post_tasks are printing config_names value from test.yml as expected.

Execution 2:

  ansible-playbook -i test.yml playbook.yml

Result:
Prompts for filename.
pre_tasks and post_tasks are printing config_names value from test.yml
roles is using value from vars/static.yml file.

Summary

Some how the workflow is missing to consider prompted variable values for roles section ( not sure of underneath task ). My case is more complex, I have split up prompt and tasks into two different playbooks and included in sequential order in top playbook. The first playbook prompts and set variables using set_fact task. The second playbook would be called after executing the first, so as expected the verbose shows of loading right file then using right values in pre and post tasks. Roles failed to recognize the overridden value.

Cheers
Venkat Vagga